Modeling and optimization of enantioseparation by capillary electrochromatography

Computer simulation of electrophoretic aspects of enantiomer migration and separation in capillary electrochromatography with a neutral selector

A computer simulation study describing the electrophoretic separation and migration of methadone enantiomers in presence of free and immobilized (2-hydroxypropyl)-β-CD is presented. The 1:1 interaction of methadone with the neutral CD was simulated by using experimentally determined mobilities and complexation constants for the complexes in a low-pH BGE comprising phosphoric acid and KOH. The use of complex mobilities represents free solution conditions with the chiral selector being a buffer additive, whereas complex mobilities set to zero provide data that mimic migration and separation with the chiral selector being immobilized, that is CEC conditions in absence of unspecific interaction between analytes and the chiral stationary phase. Simulation data reveal that separations are quicker, electrophoretic displacement rates are reduced, and sensitivity is enhanced in CEC with on-column detection in comparison to free solution conditions. Simulation is used to study electrophoretic analyte behavior at the interface between sample and the CEC column with the chiral selector (analyte stacking) and at the rear end when analytes leave the environment with complexation (analyte destacking). The latter aspect is relevant for off-column analyte detection in CEC and is described here for the first time via the dynamics of migrating analyte zones. Simulation provides insight into means to counteract analyte dilution at the column end via use of a BGE with higher conductivity. Furthermore, the impact of EOF on analyte migration, separation, and detection for configurations with the selector zone being displaced or remaining immobilized under buffer flow is simulated. In all cases, the data reveal that detection should occur within or immediately after the selector zone.

Analysis of phenolic xenoestrogens by pressurized CEC with amperometric detection

A new method, pressurized CEC with end-column amperometric detection using carbon paste electrode, has been developed for the separation and determination of five phenolic xenoestrogens in chicken eggs and milk powder samples. Efficient separation of five analytes was performed by pressurized CEC using a mobile phase consisting of 60% v/v ACN and 40% v/v Tris buffer (5 mmol/L, pH 8.0), +6 kV of applied voltage and 7.0 MPa of supplementary pressure. Detection limits of 50, 5, 2, 10 and 20 ng/mL for pentachlorophenol, bisphenol-A, 2,4-dichlorophenol, 4-tert-octylphenol and 4-nonylphenol, respectively, were achieved using carbon paste electrode as working electrode and +0.8 V as detection potential. Matrix solid phase dispersion extraction method had been employed during sample preparation procedure, and mean recoveries ranged from 79.2 to 102.6% at different concentrations of phenolic xenoestrogens for spiked egg and milk powder samples were obtained.

Computational Modeling of Inclusion Complexes of β-Cyclodextrin with enantiomers of Salsolinol, N-Methyl-Salsolinol, and 1-Benzyl-Tetrahydroisoquinoline

Capillary electrophoresis with β-CD as a chiral selector has successfully separated the two enantiomers of salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline. The migration times of each enantiomer in capillary electrophoresis reflect the stability of their β-CD inclusion complexes. This paper reports a computational modeling study of the inclusion complexes of β-cyclodextrin (β-CD) with salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline by using PM3 (Parametric Method 3) semi-empirical molecular orbital calculations and the ONIOM hybrid method. Two types of the inclusion complexes, cis- and trans-orientations, are considered for each enantiomer of the guest molecules, salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline. In the cis-orientation, the nitrogen in the salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline points toward the secondary hydroxyls of the β-CD, while in the trans-orientation, the nitrogen in salsolinol, N-methyl-salsolinol, and 1-benzyl-tetrahydroisoquinoline points toward the primary hydroxyls of the β-CD. We found that the stabilization energies of these inclusion complexes from these PM3 and ONIOM different methods correlate very well with the migration order deduced from the study of capillary electrophoretic separation.

On-Column Coaxial Flow Chemiluminescence Detection for Underivatized Amino Acids by Pressurized Capillary Electrochromatography Using a Monolithic Column

A new method for pressurized capillary electrochromatography (pCEC) coupling with chemiluminescence (CL) detection using a modified on-column coaxial flow detection interface was developed. To evaluate the feasibility and reliability of the experimental setup, the typical CL compounds luminol and isoluminol were separated and detected by using this pCEC-CL system. A detailed investigation of CL detection interface and postcolumn CL reagent flow rate parameters was described. The excellent resolution and detection sensitivity was achieved by using 3-microm ODS-C18 packed column with 30% ACN (v/v), 5 mmol/L phosphate buffer (pH 8.0). Moreover, with the presence of Co(II) (1.0 x 10(-4) mol/L) in the mobile phase, the linear range of the concentration for luminol was 2.0 x 10(-9)-2.0 x 10(-6) mol/L with a detection limit (S/N = 3) of 2.0 x 10(-10) mol/L, and 2.5 x 10(4) theoretical plates was achieved. In addition, separation and detection of the underivatized amino acids (l-threonine and l-tyrosine) were accomplished by using a polymerized monolithic column based on the principle of the luminol-H2O2-Cu(II)-amino acid CL system. Under the optimum conditions, the mixture of amino acids was efficiently separated with satisfactory results.

Effect of the Net Surface Charge Density of Heptakis-6-bromo-6-deoxy-.BETA.-cyclodextrin Bonded Silica Gels on the Retention Behaviors of Neutral Cresol Isomers in HPLC

The effect of the surface charge density of heptakis-6-bromo-6-deoxy-beta-cyclodextrin (beta-CD-BR) bonded silica gels, which was used as the stationary phase of a packed capillary column for HPLC, was investigated concerning the retention behaviors of neutral cresol isomers. On the whole, the retention factors of the cresol isomers increased with an increase in the pH values of the mobile phase, although they were slightly smaller at pH 6.1 than at pH 4.7. An investigation on the retention variation using a van't Hoff plot revealed that the increase in the retention factor (k) at a higher pH region could be mainly attributed to the increase in DeltaS, while a partial decrease in k around pH 5 - 6 was caused by a decrease in the -DeltaH/T value. On the other hand, a measurement of the electroosmotic flow velocity under various pH of the mobile phase solutions revealed that the retention variations of the neutral cresol isomers were strongly correlated with the surface charge on the packing materials. The positive charge of secondary ammonium functional groups to bind beta-CD-BR inhibit the insertion of the cresol isomers into the cavity of beta-CD-BR while reducing the retention factor, whereas the negative charge of silanol group enhanced it through a local change in the mobile phase composition.

Capillary electrochromatographic chiral separations with potential for pharmaceutical analysis

The use of capillary electrochromatography as a chiral separation technique for pharmaceutical applications is reviewed. Publications of the past 10 years that provide a potential practical application in pharmaceutical analysis are considered. Method development or validation, separation strategies, and potential routine analysis by the methods/applications cited are the main subjects on which we focused our attention. The indirect chiral separation method was only used once in CEC mode. In the direct chiral separations, the use of chiral stationary phases was obviously preferred over the use of chiral mobile phases with non-chiral stationary phases. Amongst the chiral stationary phases, those based on macrocyclic antibiotics and polysaccharide selectors were the most frequently used. Monolithic stationary phases also have several applications, but not so extended as those with packed capillary electrochromatography. The considered papers not only describe the applicability of the technique for relatively large sets of chiral analytes, they also showed that various types of stationary phases can be produced in-house in a simple manner. However, to survive as a mature separation technique, considerable time and effort are still needed to solve some disadvantages currently characterizing capillary electrochromatography.

Chiral CE Separation of Dopamine-Derived Neurotoxins

An enantiomeric separation of dopamine-derived neurotoxins by capillary electrophoresis has been developed. Tetrahydroisoquinoline (TIQ), dopamine (DA), (R/S)-1-benzyl-TIQ (BTIQ), (R/S)-6,7-dihydroxy-1-methyl-TIQ (salsolinol, Sal), and (R/S)-6,7-dihydroxy-1, 2-dimethyl-TIQ (N-methyl-salsolinol, NMSal) were studied as model compounds. The CE running buffer (50 mM phosphate buffer at pH 3.0) contained 1.5 M urea and 12 mM beta-CD as a chiral selector. During separation, the (R)-enantiomers formed more stable inclusion complexes with beta-CD, and thus had a longer migration time than their optical antipodes. It was noticed that the recovery rates of these TIQ derivatives were very poor (< 15%) during protein precipitation, a procedure widely used for cleaning up biological samples. The recovery was significantly improved by pre-mixing the sample with a surfactant (e.g., sodium hexanesulfonate or Triton X-100) to reduce the co-precipitation. The present method in combination with electrospray ionization tandem mass spectrometry (ESI-MS/MS) was applied to study samples obtained from in vitro incubation of two catecholamines, dopamine and epinine, with aldehydes forming neurotoxins including (S)- and (R)-NMSal enantiomers. The later is known to induce Parkinsonism in rats.

Comparison of column performances in direct high-performance liquid chromatographic enantioseparation of 1- or 3-methyl-substituted tetrahydroisoquinoline analogs. Application of direct and indirect methods

The enantioseparability of 1- or 3-methyl-substituted tetrahydroisoquinolines was investigated by direct and indirect high-performance liquid chromatography. Alpha- and beta-cyclodextrin-, macrocyclic glycopeptide- and cellulose-based chiral columns and isothiocyanate- and Sanger-type chiral derivatizing agents (CDAs) were applied to attain satisfactory enantioseparation. Of the chiral columns, beta-cyclodextrin-, vancomycin- and teicoplanin-containing macrocyclic glycopeptide-based columns appeared to be most suitable; of the CDAs, (1S,2R)-1-acetoxy-1-phenyl-2-propyl isothiocyanate was most favorable. The differences between the selectivities of the columns and CDAs were compared and optimized to yield the best resolution.

Analytical method for the quantitation of sertraline hydrochloride stereoisomers by electrokinetic chromatography

Sertraline is a basic compound and of pharmaceutical application for antidepressant treatment. The compound has two chiral centers. Separation of the three enantiomeric impurities from the parent compound is challenging. In this study, we successfully separated all four stereoisomers by electrokinetic chromatography using highly sulfated gamma-cyclodextrin and highly sulfated alpha-cyclodextrin as the chiral selectors. The two chiral selectors provided different selectivity and therefore affected the overall separation profiles. This may be due to the size difference between the dichlorophenyl moiety end and naphthalenamine moiety end, resulting in two different types of inclusion complexes with the different cyclodextrins. For routine analysis, highly sulfated gamma-cyclodextrin was better than highly sulfated alpha-cyclodextrin. For each stereoisomeric impurity, the method using sulfated gamma-cyclodextrin provided a limit of quantitation at or lower than 0.1% of the drug substance with adequate resolution. The critical resolution at this concentration level was not less than 4.0. Experimental data suggested that an internal standard was necessary for the purpose of quantitation, and the practical linearity range for analysis of sertraline stereoisomeric impurities was of about two orders of magnitude.

Characterization of open tubular capillary electrochromatography columns for the analysis of synthetic peptides using isocratic conditions

In this paper, we report on investigations related to the performance characteristics of two different types of etched chemically (n-octadecyl- and cholesterol-) modified capillaries in the open tubular format of capillary electrochromatography (CEC) for the analysis of synthetic peptides. The results confirm that the nature of the surface chemistry used to modify the capillary wall and type of chemically bonded group employed can affect the selectivity as well as the resolution of peptide samples. The results are consistent with the participation of selective peptide interactions with the bonded phase, although other factors, such as the morphology of the capillary wall surfaces, appear to be also involved. Moreover, several surprising observations related to peptide-specific multi-zoning effects have been observed. Additional experimental variables that can also be utilized to affect the retention of peptides in this approach to OTCEC include the type and percentage of organic solvent modifier employed in the eluent and the pH of the buffer system. To evaluate the reproducibility of different batches of the n-octadecyl- and cholesterol-modified capillaries and the stability of the chemically modified surface, the OTCEC selectivity and peak shape behavior of two small basic molecules (serotonin and tryptamine) and two proteins (turkey and chicken lysozyme) were also investigated. Finally, the use of the "bubble" cell technology for creating the detector window has been shown to provide significantly higher detection sensitivity with peptides, as compared with the conventional capillary format.

Separation of enantiomers by open capillary electrochromatography on polysiloxane-bonded permethyl-beta-cyclodextrin

The separation of enantiomers by open capillary electrochromatography (o-CEC) using Chirasil-Dex as chiral stationary phase (CSP) is reviewed. In Chirasil-Dex, permethylated beta-cyclodextrin is linked via a single octamethylene spacer to polydimethylsiloxane. The CSP is coated and thermally immobilized onto the internal surface of a fused-silica column (i.d. 50 microm). Employing a single open-tubular column coated with Chirasil-Dex, a unified enantioselective approach can be realized using the four common chromatographic techniques: o-GC, o-SFC, o-LC and o-CEC. The chiral stationary phase Chirasil-Dex can be combined with a charged cyclodextrin derivative, which is added into the mobile phase. In the resulting dual chiral recognition system, enhancement of enantioselectivity (matched case) or compensation of enantioselectivity (mismatched case) are observed. The overall enantioselectivity is dependent on the sense of enantioselectivity of the selectors chosen and their influence on the electrophoretic and electroosmotic migration of the enantiomers of a selectand. The feasibility to couple chiral o-CEC and ESI/MS is demonstrated for trace analysis of enantiomeric drugs in body fluids.

Stationary phases for capillary electrophoresis and capillary electrochromatography

An overview of the most recent developments in column technology employed in capillary electrophoresis (CE) and capillary electrochromatography (CEC), mainly for the separation of small molecules and ions, is presented. Particular emphasis is laid on permanent coating. The wall modification methods in CE include covalent modification, adsorbed coatings and polymeric coatings, while those in CEC include packed columns, open-tubular columns and fritless columns. A short discussion on the characterization and selectivity of the bonded phases is also given.

Enantioseparations in capillary electromigration techniques: recent developments and future trends

This review summarizes the current status of enantioseparations using capillary electromigration techniques and gives the authors insights on the selected fundamental aspects and future trends in this field. The most recent developments in the field of chiral separations using capillary electrophoresis (CE) and capillary electrochromatography (CEC) are summarized. The status of chiral electromigration techniques is evaluated tacking into account the most recent developments in related techniques such as chiral HPLC, GC and SFC.

Recent progress in enantiomer separation by capillary electrochromatography

Enantiomer separation by electrochromatography (CEC) can be performed in three modes: (i) open-tubular capillary electrochromatography (o-CEC), in which the chiral selector is physically adsorbed coated, and thermally immobilized or covalently attached to the internal capillary wall; (ii) packed capillary electrochromatography (p-CEC), in which the capillary is either filled with chiral modified silica particles or with an achiral packing material, and a chiral selector is added to the mobile phase; and (iii) monolithic (rod)-capillary electrochromatography (rod-CEC) in which the chiral stationary phase (CSP) consists of a single piece of porous solid. We present an overview on methods and new trends in the field of electrochromatographic enantiomer separation such as CEC with either nonaqueous mobile phases or stationary phases with incorporated permanent charges, or with packing beds consisting of nonporous silica particles or particles with very small internal diameters.

Recent progress in chiral separation principles in capillary electrophoresis

This review summarizes recent developments in the field of chiral separations by electromigration techniques including capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), isotachophoresis (ITP), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC). This overview focuses on the development of new chiral selectors and the introduction of new techniques rather than applications of already established selectors and methods. The mechanisms of the different chiral separation principles are discussed.

Stationary phases for capillary electrochromatography

This review summarizes the variety of stationary phases that have been employed for capillary electrochromatography (CEC) separations. Currently, about 70% of reported CEC research utilizes C18 stationary phases designed for liquid chromatography, but an increasing number of new materials (e.g., ion-exchange phases, sol-gel approaches, organic polymer continuous beds) are under development for use in CEC. Novel aspects of these different materials are discussed including the ability to promote electroosmotic flow, phase selectivity and activity for basic solutes. In addition, new column designs (polymer continuous beds and silica-sol-gel monoliths) are described.

Chiral anion exchangers applied to capillary electrochromatography enantioseparation of oppositely charged chiral analytes: investigation of stationary and mobile phase parameters

Weak anion-exchange (WAX) type chiral stationary phases (CSPs) based on tert.-butyl carbamoyl quinine as chiral selector (SO) and different types of silica particles (porous and non-porous) as chromatographic support are evaluated in packed capillary electrochromatography (CEC). Their ability to resolve the enantiomers of negatively charged chiral analytes, e.g., N-derivatized amino acids, in the anion-exchange mode and their electrochromatographic characteristics are described in dependence of several mobile phase parameters (pH, buffer type and concentration, organic modifier type and concentration) and other experimental variables (electric field strength, capillary temperature). The inherent "zwitterionic" surface character of such silica-based WAX type CSPs (positively charged SO and negatively charged residual silanols) allows the reversal of the electroosmotic flow (EOF) towards the anode at pH values below the isoelectric point (pI) of the modified surface, whereas a cathodic EOF results at pH values above the pI. Since for negatively charged analytes also an electrophoretic transport increment has to be considered, which can be either in or against the EOF direction, several distinct modes of elution have been observed under different stationary phase and mobile phase conditions: (i) co-electrophoretic elution of the negatively charged solutes with the anodic EOF in the negative polarity mode, (ii) counter-electrophoretic elution with the cathodic EOF in the positive polarity mode, and (iii) electrophoretically dominated elution in the negative polarity mode with a cathodic EOF directed to the injection end of the capillary. Useful enantioseparations of chiral acids have been obtained with all three modes. Enantioselectivity values as high as under pressure-driven conditions and theoretical plate numbers up to 120000 per meter could be achieved under electrically driven conditions. A repeatability study yielded RSD values below 2% for retention times and RSD values in the range of 5-10% for theoretical plate numbers and resolution, thus clearly establishing the reliability of the investigated anion-exchange type CEC enantioseparation methods.

Open tubular capillary electrokinetic chromatography in etched fused-silica tubes

This review describes an open tubular approach to capillary electrochromatography (OTCEC) that first etches the inner surface of the fused-silica tube using ammonium hydrogen diflouride. This process can increase the inner surface area significantly. The new surface is then chemically modified to attach a bonded stationary phase using a silanization/hydrosilation reaction process. The surfaces are characterized spectroscopically by diffuse reflectance infrared Fourier transform and by electroosmotic flow measurements. Applications of OTCEC columns with C18, diol and chiral stationary phases are described.

Enantioselective determination by capillary electrophoresis with cyclodextrins as chiral selectors

This review surveys the separation of enantiomers by capillary electrophoresis using cyclodextrins as chiral selector. Cyclodextrins or their derivatives have been widely employed for the direct chiral resolution of a wide number of enantiomers, mainly of pharmaceutical interest, selected examples are reported in the tables. For method optimisation, several parameters influencing the enantioresolution, e.g., cyclodextrin type and concentration, buffer pH and composition, presence of organic solvents or complexing additives in the buffer were considered and discussed. Finally, selected applications to real samples such as pharmaceutical formulations, biological and medical samples are also discussed.

Enantiomer separation of drugs by capillary electromigration techniques

The review summarizes the most recent developments in the field of enantioseparation of chiral drugs using capillary electromigration techniques. The basic principles of enantioseparations in CE are discussed. Recent developments in sample introduction, separation and detection in capillary electrophoresis and capillary electrochromatography are summarized. The applications are arbitrarily divided into the following three groups: (a) racemates and artificial mixtures of enantiomers, (b) drug forms and (c) chiral drugs and their metabolites in biological fluids. Among the various techniques involved the relatively new developments such as CEC in aqueous and nonaqueous buffers, on-line CE-MS coupling, etc. are emphasized.

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography (CEC) is achieved with open-tubular capillaries (o-CEC), with packed capillaries (p-CEC) or with monolithic capillaries. In o-CEC, capillaries are coated with a thin film containing cyclodextrin derivatives, cellulose, proteins, poly-terguride or molecularly imprinted polymers as chiral selectors. In p-CEC, typical chiral HPLC stationary phases such as silica-bonded cyclodextrin or cellulose derivatives, proteins, glycoproteins, macrocyclic antibiotics, quinine-derived and 'Pirkle' selectors, polyacrylamides and molecularly imprinted polymers are used as chiral selectors. Chiral monolithic stationary phases prepared by in situ polymerization into the capillary were also developed for electrochromatographic enantiomer separation.

Immobilisation and evaluation of a vancomycin chiral stationary phase for capillary electrochromatography

The macrocyclic antibiotic, vancomycin, is covalently bonded to LiChrospher diol silica packed columns and evaluated in capillary electrochromatography (CEC) both in the reversed-phase and polar organic mode. Initially, capillaries were packed with 5 microm LiChrospher 100 A diol silica and evaluated in CEC with a reversed-phase biphenyl-pyrene achiral test resulting in resolution and efficiency values of ca. 2.5 and 100000 plates meter(-1), respectively. Repeatability for this test (resolution and efficiency) was also examined and found to be acceptable for both run-to-run (n=5, 0.74% and 1.5%) and column-to-column (n=5, 3.4% and 9.0%), respectively. Similar results were obtained when the 10 microm LiChrospher 1000 A diol silica was examined with the exception of efficiency, where a reduced plate height value of four times lower was obtained compared to the 100 A material. A simple three step in-situ vancomycin immobilisation procedure was subsequently carried out on these packed diol columns. Selectivity was obtained for thalidomide enantiomers on this vancomycin chiral stationary phase in reversed-phase CEC with resolution and efficiency values of ca. 2.5 and 80000 plates meter(-1), with acceptable repeatability (n=8) 0.9% and 3.0%, respectively. Selectivity was also obtained for thalidomide enantiomers on this phase in the polar organic mode with resolution and efficiency values of ca. 2.5 and 120000 plates meter(-1), with acceptable repeatability (n=7) 0.9% and 2.0%, respectively. It was possible to deduce from a chemometric design carried out for evaluating the mobile phase component effects that organic modifier ratio, MeOH/MeCN, played a significant role in controlling both resolution and efficiency. It was also possible to separate a number of basic analytes including four beta-adrenergic blocking agents in the polar organic mode albeit with lower resolution and efficiency values, ca. 1.5 and 45000 plates meter(-1), respectively.

Silicate polymerization for the preparation of bed-retention frits in capillary electrochromatography

Bubble formation, which is associated with bed-retention frits, is a critical experimental problem in capillary electrochromatography systems. In this investigation, porous silica frits were prepared via spot-heating of a silicate solution, and the effects of several experimental parameters on their performance were studied. The optimal sodium silicate concentrations were 10.8% and 5.4% (w/v) for outlet and inlet frits, respectively. The heating times were 5-6 s for outlet frits and < 1 s for inlet frits. Under optimized conditions, outlet frits were 75 microns (+/- 12 microns) and the heat treatment did not make the capillary fragile at the frit location. Bubble formation was affected by frit length, density, and silanization of the frits with trimethylchlorosilane. Packed capillaries with optimized frits were used successfully in a commercial CE instrument over a normal working day without pressurization, at relatively high ionic strengths (10 mM), and over a wide range of acetonitrile compositions (20%-80%). Currents were also stable for > or = 3 h under very high current (27 microA) conditions. As part of this study, the efficiency and reproducibility of packed capillaries were also briefly evaluated.

Applications of capillary electrochromatography

Applications performed by capillary electrochromatography (CEC) in all its modes, namely packed column CEC (packed-CEC), open tubular CEC (OT-CEC) and pressure-assisted CEC (pseudo-CEC), and published by June 1999 are reviewed. The review is divided into (i) separation of neutral, acidic and basic analytes with the main goal of evaluating column and system performance, (ii) separation according to field of application and/or chemical class, and (iii) separation of chiral analytes.

Enantiomer separation by pressure-supported electrochromatography using capillaries packed with Chirasil-Dex polymer-coated silica

Pressure-supported electrochromatography using capillaries packed with permethyl-cyclodextrin covalently linked via an octamethylene spacer to dimethylpolysiloxane and immobilized on silica (Chirasil-Dex silica) has been employed as an efficient and rapid method for the enantiomer separation of various racemic compounds. By comparing this method with micropacked liquid chromatography (LC), employing the same column in a unified instrumental setup, micropacked capillary electrochromatography (CEC) shows higher column efficiencies and hence better resolution factors. The influence of type and concentration of buffer, amount and nature of organic modifier, and pressure support is investigated.

Recent innovations in enantiomer separation by electrochromatography utilizing modified cyclodextrins as stationary phases

Enantiomer separation by electrochromatography employing modified cyclodextrins as stationary phases is performed in two ways. (i) Polysiloxane-linked permethylated beta-cyclodextrin (Chirasil-Dex 1) or related selectors are coated and immobilized onto the inner surface of a capillary column. Enantiomer separation is performed in the open tube and the method is referred to as open-tubular capillary electrochromatography (o-CEC). (ii) Silica-linked native beta-cyclodextrin, permethylated beta-cyclodextrin (Chira-Dex 2) or hydroxypropyl-beta-cyclodextrin are filled into a capillary column and the bed is secured by two frits. Enantiomer separation is performed in a packed column and the method is referred to as packed capillary electrochromatography (p-CEC). In a unified instrumental approach, method (i) as well as method (ii) can be operated both in the electro- and pressure-driven modes (o-CEC vs. open-tubular liquid chromatography (o-LC) and p-CEC vs. p-LC). It is demonstrated that the electro-driven variant affords higher efficiencies at comparable elution times. Employing a single open-tubular column coated with Chirasil-Dex 1, a unified enantioselective approach can be realized in which the same selectand is separated using all existing chromatographic modes for enantiomers, i.e., gas chromatography (GC), super-critical fluid chromatography (SFC), o-LC and o-CEC. As the chiral selector is utilized as a stationary phase, an additional chiral selector may be added to the mobile phase. In the resulting dual chiral recognition systems, enhancement of enantioselectivity (matched case) or compensation of enantioselectivity (mismatched case) may be observed. The overall enantioselectivity is dependent on the sense of enantioselectivity of the selectors chosen and their influence on the electrophoretic and electroosmotic migration of the enantiomers of a selectand.